First, you need to buy/get flash with which you can change output, best through aperture marks. After that is simple. For example you use ISO 100 film, 1/125 shutter speed, f11 stopped lens. Transfer same parameters to your flash, that is ISO 100 for film (and distance to your subject if your flash is totally manual), but set your flash to range to "tell" flash like you use lens set to between f8 to f5,6 (that is one to two stops more open then your lens is actually open).
Flash will think you use lens set to f8, and will output less light (one stop less instead what is needed for f11 (how you actually set you lens)). If that is too much light, set flash to f5,6 stopped lens, but leave lens set to f11.
If you can't have flash with output changing option, that is if your flash have fixed output power, then you need to improvize. For example, place layers of white tissue over flash head to reduce output, move flash away from your subject, etc...
Other way is to use camera/flash system capable to do that.
For example I have flash without changing output on flash (Canon 420EX). But, because it is system flash, I set my lens for example to f11 and on camera (Canon EOS3) there is command to change flash output. That way if I want fill flash I on camera set flash to -1 stop (down to -2 depend of sitation) less flash than is really needed for lens set on f11, and get automatically fill flash because TTL metering calculate subject distance and (in this case reduced) flash power.
But, for mine Mamiya RB, Bessa R2A, Yashica MAT I am buying manuall controlable flash too